This invention relates to a graticule illumination system for use with a night sight including an image intensifier. More particularly, it relates to a graticule illumination system for use with an image intensifier having a phosphor screen on which an intensified output image is displayed in a first colour characteristic of the phosphor. The illumination system comprises means for superimposing a graticule image effectively onto the phosphor screen and for illuminating the graticule with light of a second colour contrasting with the first colour.
Such a graticule illumination system is described in British patent application No. 2,090,012A in which an eye piece magnifying lens system used to view the phosphor screen includes a semi-reflective layer at an angle of 45 degrees to the eye piece axis and arranged to present a virtual image of an illuminated graticule in coincidence with the phosphor screen. If the graticule illumination is removed, the graticule disappears altogether. Also, to maintain adequate visibility of the graticule throughout the range of phosphor screen brightness obtained when observing scenes illuminated with overcast starlight up to moonlight, the graticule illumination must be adjusted by the user to suit the current screen brightness. Care is required in making this adjustment to ensure visibility of the graticule at high definition without introducing glare which might obscure target details. Under battlefield conditions there may not be time to set the graticule brightness to a value suited to the brightness of the target momentarily being observed.
Alternatively, an opaque graticule pattern may be used effectively in coincidence with the phosphor screen. At high scene brightnesses, and hence high screen brightness, the graticule will be visible as a black pattern, much as in a conventional telescopic sight. However at low screen brightnesses the visibility of the graticule declines and eventually becomes unuseable at the lowest scene brightnesses.